Most individuals equate feeding horses to gain muscle or to achieve high performance with feeding protein. But there is much more to the protein equation than just how much is consumed in the diet. Muscle protein synthesis is regulated by a system called mTOR, which itself is activated by dietary amino acids, insulin and exercise, each acting in a different manner to activate mTOR. Young growing horses experience a greater response in muscle protein synthesis to meal feeding (which makes sense as they are still growing) than do mature horses. However, this response was maximized at feeding 0.25 g protein/kg body weight. To put this in relative terms, a 700 pound yearling would not experience further activation of mTOR once consuming 1.25 pounds of a 14% CP feed in one meal.
Practically, this means that ideally the high protein portion of a horse’s diet should be provided in multiple meals throughout the day, rather than a single feeding. Unfortunately, individual protein sources or supplemental amino acids in the horse’s diet have yet to be tested relative to differences in mTOR activation. In general, concentrate sources of protein are more readily digestible and available than forage sources, presumably resulting in greater protein synthesis activation.
Protein is also commonly fed to horses requiring more energy, despite the fact that carbohydrates and fats are the primary fuel sources for muscle contraction. During low intensity exercise, glucose and fatty acid metabolism supply energy through aerobic ATP generation. During high intensity exercise a greater shift to glucose metabolism occurs. High intensity exercise is therefore more reliant on utilization of muscle glycogen stores. Utilization of fat added diets in performance horses shifts their reliance of energy substrate to greater use of fatty acid metabolism, saving carbohydrate stores for bouts of more intense exercise. However, adaptation to fat in the diet should take place over several weeks, to avoid gastrointestinal upsets.
The enzyme systems which utilize fatty acids also need time to adapt and up regulate to a greater reliance on fat metabolism. Finally, if using added fat as a strategy to enhance muscle glycogen, adequate carbohydrates from grains must still be included in the diet. Amino acids, while they can be catabolized for fuel, are not a primary source of energy with the possible exception of prolonged exercise. Instead, it is more important to supply key amino acids to aid in muscle recovery and repair. Providing supplemental glucose to the horse post exercise does not appear to enhance muscle glycogen repletion, contrary to what is observed in people. To date, there does not appear to be an effective strategy to increase the speed of glycogen recovery in the horse, which may take as long as 48-72 hours to achieve.
Unfortunately, despite the prevalence of many supplements and advertised feeding strategies to enhance muscle accretion, no validated research has been able to bear this out. Additional amino acids beyond the horse’s requirements cannot stimulate protein synthesis without an accompanying strategic exercise program. In general, if mature horses are consuming diets of at least 10-12% total protein which match their energy intake needs, they are receiving adequate protein. The exact amino acid requirements of horses are still relatively unknown, and more research is needed on products which indicate performance enhancements.
See more at OSU equine news.
